Microwave ovens often provide a quick and convenient way of cooking and heating food substances. Microwave ovens typically heat food substances more quickly than a conventional oven. In some instances, for example, a product which must be cooked for 30 minutes in a conventional oven may be cooked in a microwave oven in 4 minutes or less.
However, microwave energy cooks foods differently from a conventional oven. In a conventional oven, the high temperature atmosphere impinges on the surface of the food substance, causing the surface to heat first. Moisture is driven from the exterior surface of the food substance by the hot oven atmosphere, and this often results in a crisp exterior surface of the food substance. Initially a temperature gradient is established where the center of the food substance is cool, and the exterior surface is elevated in temperature by the heat of the oven. The movement of moisture is affected by the nature of the temperature gradient. Other heat transfer mechanisms may also be at work, e.g., radiation from a heat source. But such mechanisms result in heating that initially starts at the surface and progresses relatively slowly toward the center of the food substance. Transfer of heat to the center of the food substance is by conduction and possibly other heat transfer mechanisms. Moisture migration in a conventional oven environment is normally conducive to achieving a crisp exterior surface.
A microwave oven, on the other hand, generates high intensity, high frequency electromagnetic radiation which penetrates into a food substance. Heating occurs when the electromagnetic energy is absorbed by the food substance. Different food substances, and different layers of the same food item, may absorb different amounts of microwave energy. The amount of heating depends upon the strength of the electric field as it penetrates a particular layer of the food, and the tendency of that layer to absorb microwave energy. In most cases, the heating effects of microwave energy penetrate to a much greater depth toward the center of the food substance than is the case with a conventional oven. The center of a food substance will be heated much more quickly. In sharp contrast to the situation which may exist in a conventional oven, where the surface of the food substance is heated to a high temperature, in a microwave oven a breaded and battered surface is rarely heated sufficiently to crisp it.
Although the surface of a battered and breaded food product may be in a high intensity field, the tendency of that layer to absorb microwave energy is too low to cause it to be elevated to a sufficiently high temperature to result in a crisp surface. To make matters worse, moisture is typically driven from the interior of a high moisture content food substance, such as fish, when the interior of the food substance is rapidly heated by microwave energy. The surface, if it is not heated sufficiently to drive this moisture away, will end up with too much moisture to achieve desirable crispness.
In any event, it will be appreciated that the heat gradient set up in a microwave oven will often differ dramatically from that of a conventional oven. These differences dramatically affect the taste and substance of some foods to the point where microwave cooking of such foods has resulted in unacceptable food quality.
In the past, uneven heating of food substances in microwave ovens may have been observed. However, there has been little or no appreciation for why such uneven heating occurs in microwave ovens. There have been general efforts to avoid uneven heating by rotating food substances in the microwave oven during irradiation. And even if there has been some appreciation of some of the mechanisms causing uneven heating phenomenon, and the recognition that standing waves exist, there has been little or no appreciation of how such mechanisms can be advantageously applied to achieve desirable heating effects which heretofore have been unobtainable in microwave heating. In the past, there has been little or no recognition that the food substance can be positioned in a standing wave pattern to advantageously adjust the energy balance during microwave cooking.
In the past, food products such as breaded fish, breaded chicken, breaded vegetables, etc. have not been satisfactorily cooked in microwave ovens. In such products, it is desirable to have a crisp exterior surface. A crisp exterior surface is accomplished in a conventional oven where heating occurs from the impingement of a hot oven atmosphere to elevate the temperature of the surface of the food. In a microwave oven, however, the surface of the food substance is typically heated insufficiently by microwave absorption alone. It has been difficult in the past to achieve a crisp exterior surface in a microwave oven.
The hot oven atmosphere and temperature gradient established by a conventional oven tends to drive moisture from the surface of a breaded food product. The surface layers are initially rapidly raised to a higher temperature than the interior of a food product, which tends to enhance the crispness of the surface. This crispness has an important effect upon the sensory perception of a person who eats the food product. A breaded food product having a mushy surface tends to give a dramatically different and unacceptable taste sensation as compared with an otherwise identical food product that is crisp. The temperature characteristics of microwave heating tend to result in moisture being driven from the center of the food product to the surface, and inadequate heating of the surface to reduce the moisture content of the breaded surface to a sufficiently low level to be perceived as "crisp." Thus, the achievement of a crisp exterior surface in a microwave oven, especially in the case of breaded food products like fish which have a high moisture content, has been a problem in the past. Prior art attempts to obtain a crisp surface have been unsatisfactory.
Proper microwave cooking of food products to achieve a crisp surface involves a somewhat complex energy balance. For example, it is conceivably possible to continue cooking a breaded food product such as fish in a microwave oven long enough to crisp the exterior surface. However, this would normally result in an overcooking of the interior of the fish. An attempt could be made to increase the heating of the breaded and battered surface of the fish by increasing the amount of microwave energy that is absorbed either by increasing the cooking time or by increasing the power of the oven. But this would simultaneously increase the amount of microwave energy that is absorbed by the interior of the fish product to the point that the fish itself would be overcooked. This energy balance imposes constraints upon attempts to manipulate of the amount of microwave energy that is absorbed by the surface of the food. Increasing the cooking time or the power level of the microwave energy in order to crisp the exterior surface of the food substance is an unsatisfactory solution to the problem. Due to the cooking characteristics of microwave energy, in the example of breaded and battered fish products, it is desirable to slow down the heating of the interior of the fish and to increase the amount of heating of the exterior surface of the fish. Discovering how to do this has been a problem.
Microwave cooking must also deal with a much shorter moisture migration time. In a conventional oven, moisture migration from the center of the fish to the surface and evaporation into the oven atmosphere may occur over a 30 minute cooking period. In a microwave oven, the same fish fillet would be cooked in 31/2 to 4 minutes. The heating process occurs much more quickly, and the moisture that is going to be released tends to pour out in a small amount of time. The breading coating does not absorb enough microwave energy to get itself hot enough to deal with all of the moisture that comes out of the fish, in order to vaporize the moisture or otherwise reduce the average moisture content sufficiently to result in a crisp surface. Thus, one of the very reasons that microwave cooking is convenient, i.e., rapid cooking time, is also a significant part of the problem of crisping food surfaces--it provides a much shorter moisture movement time. Achieving a crisp surface in such a short moisture movement time in a high moisture content food has been a problem in the past.
A crisp food product would seem to require crisping on all sides of the food product. One might think that crisping of breaded fish and the like in a microwave oven would at least require some means for flipping the fish over midway through the heating process. Alternatively, one might think that the only solution to the problem of crisping breaded fish would require some mechanism for simultaneously crisping all sides of a fish stick. U.S. Pat. No. 4,267,420, issued to Brastad, and U.S. Pat. No. 4,230,924, issued to Brastad et al., are examples of attempts to produce flexible wrapping material which was wrapped completely around a fish stick to brown the surface of the fish stick. Flexible wrapping material cannot be used as a self supporting heating platform. Moreover, surrounding a food substance with wrapping material tends to contain moisture which can give the food an overall impression of sogginess, especially where the wrapper material is relatively impermeable to moisture.
The need for a crisp surface should not be confused with prior attempts to accomplish "browning" of a food substance in a microwave oven. Browning is a different concept from crispness. Browning may involve placing grill marks or otherwise discoloring the surface of a food substance in an attempt to simulate the effects of a hot grill or radiation type heating such as broiling. Browning is concerned with the appearance of the food. "Crispness" involves obtaining certain physical qualities in the surface of the food substance so that the food product will produce a taste sensation characteristic of a crisp food product. Whereas "browning" appeals to the sense of vision, "crispness" appeals primarily to the senses of taste and touch.
One approach to solving the dilemma of producing food substances which have a crisp exterior surface is to provide a heating utensil which has at least one surface of the utensil which is a lossy heater, such as browning and crisping dishes. Some such heaters use ferrites on metals or semiconductors on ceramics as the lossy elements. Such heating utensils are permanent, nondisposable in nature, and employ heating elements that require preheating in order to work. For an example of a cooking utensil employing a lossy ceramic heater, see U.S. Pat. No. 3,941,967, issued to Sumi et al. The drawbacks of nondisposable ceramic heating elements are discussed in U.S. Pat. No. 4,283,427, issued to Winters et al. According to Winters et al., ceramic heating elements are expensive and add considerable bulk and weight to packaged products. Ceramic heating elements do not readily lend themselves to employment with disposable non-permanent packaging materials. According to Winters et al., ceramic heating elements may provide for uncontrolled (runaway) heating to elevated temperatures which can often result in scorching, charring and burning. While these types of browning and crisping dishes may have their place in microwave technology, they have considerable deficiencies for many uses.
It will be apparent from the above discussion that prior art attempts to achieve crisping of the surface of a food substance in a microwave oven have not been altogether satisfactory.